1. Field of the Invention
The invention described herein relates to data communications and, in particular, direct memory access (DMA).
2. Related Art
Remote DMA (RDMA) is a technology for transferring data from the memory of one computer or server to the memory of another, without involving a CPU or operating system of either machine. Because the data being transferred is not stored in application memory or in operating system buffers, RDMA is said to accomplish the transfer in a “zero-copy” manner.
RDMA is typically implemented using a suite of three protocols—RDMA Protocol (RDMAP), Direct Data Placement (DDP) and Marker PDU Aligned Framing Protocol (MPA). RDMAP provides interfaces to applications for sending and receiving data. DDP slices outgoing data into segments that fit into TCP's Maximum Segment Size (MSS), and places incoming data into destination buffers. MPA provides a framing scheme that facilitates DDP operations in identifying DDP segments.
RDMA is a “shim”, a transport protocol suite on top of TCP. RDMA leverages TCP rather than inventing its own protocols for flow control, routing, data sequencing and so on. In principle, an RDMA message can be too large to fit into one TCP segment.
MPA is a framing protocol. It adds a marker into the data stream at a stride of every 512 bytes in the TCP sequence space. Markers assist the receiver in locating the DDP/RDMA header.
Unfortunately, insertion and removal of MPA markers are not friendly operations. Inserting markers into a continuous data stream creates a disruptive shuffle of the data stream. Insertion and removal of a RDMA CRC (cyclic redundancy code) digest is also difficult to handle efficiently.
Therefore there is a need for a system and apparatus with which MPA markers and CRC digests can be easily inserted and removed during RDMA communications.